1. Field of the Invention
The present invention relates to a liquid crystal composition, a liquid crystal display device using the liquid crystal composition, and display method and apparatus using the liquid crystal composition and device.
More particularly, the present invention is concerned with a novel liquid crystal composition having improved electric-field response characteristic, a liquid crystal display device such as a liquid crystal-optical shutter, using the liquid crystal composition, and display method and apparatus using such a liquid crystal display device.
2. Description of the Preferred Embodiment
Use of bi-stable liquid crystal device has been proposed by Clark and Lagerwall in, for example, U.S. Pat. No. 4,367,924 and Japanese Patent Laid-Open Application No. 56-107216.
In general, ferroelectric liquid crystals having a chiral smectic C phase (SmC* phase) or chiral smectic H phase (SmH* phase) are used as bi-stable liquid crystals.
The above-mentioned ferroelectric liquid crystal has a bi-stable characteristic: namely, it can take first and second optically stable states. Therefore, the liquid crystal of this type is oriented in a first optically stable state in response to, for example, one of two electric field vectors and is oriented to a second optically stable state under the influence of the other electric field, unlike TN (Twisted Nematic) liquid crystal used in optical modulating device. In addition, this type of liquid crystal takes either one of these two stable states and maintain this state unless an electric field is applied. This feature is generally referred to as a "bi-stable" characteristic.
Ferroelectric liquid crystal also exhibits, in addition to the above-described bi-stable characteristic, a high response speed which owes to the fact that the transition of the orientation state is caused by the effects produced by spontaneous polarization of the liquid crystal itself and the electric field applied thereto. In fact, the response speed is 3 to 4 orders higher than that caused by the interaction between dielectric anisotropy and electric field.
Thus, ferroelectric liquid crystals generally have potential superiority. By using such superior characteristics, it is possible to fundamentally solve most of problems encountered by conventional TN type liquid crystal device. In particular, this type of liquid crystal is expected to find spreading use in the fields of high-speed optical shutters and high-density large-area display devices.
A simple matrix display device can be formed by sandwiching this ferroelectric liquid crystal layer between a pair of substrates. Such a display device can be driven by driving methods disclosed, for example, in Japanese Patent Laid-Open Nos. 59-193426, 59-193427, 60-156046 and 60-156047.
FIGS. 4 and 5 illustrate waveforms of signals employed in a driving method which is used in an embodiment of the present invention. FIG. 6 is a plan view of a ferroelectric liquid crystal panel 61 having matrix electrodes used in the present invention. The panel 61 shown in FIG. 6 has scanning lines 62 and data lines 63 crossing each other. A ferroelectric liquid crystal is disposed between the scanning line 62 and the data line 63 at each crossing point.
In FIG. 4, S.sub.S represents a select scanning waveform applied to a selected scanning line, while S.sub.N shows a non-select scanning waveform. I.sub.S shows a select information waveform applied to a selected data line. The information, in this case, is black. I.sub.n shows a non-select information signal which is, in this case, white, applied to a non-selected data line. In FIG. 4, (I.sub.S -S.sub.S) and (I.sub.N -S.sub.S) show voltage waveforms applied to pixels on a selected scanning line. Pixels which receive the voltage (I.sub.S -S.sub.S) show black state, while pixels which are supplied with the voltage (I.sub.N -S.sub.S) show white state.
FIG. 5 illustrate time-sequential waveforms used when information shown in FIG. 7 is displayed in accordance with the driving waveforms shown in FIG. 4.
In the driving example shown in FIGS. 4 and 5, the minimum time .DELTA.t of application of a single polarity applied to the pixels on the scanning line corresponds to the time of the writing phase t.sub.2 and the time of the 1-line clear phase t.sub.1 is set to be equal to 2 .DELTA.t.
The values of the parameters V.sub.S, V.sub.I, .DELTA.t of the driving waveforms shown in FIGS. 4 and 5 are determined by the switching characteristic of the liquid crystal material used in the panel.
FIG. 8 is a graph showing the manner in which transmittance T changes in response to a change in the driving voltage V=(V.sub.S +V.sub.I) when a bias ratio to be mentioned later is maintained constant. This characteristic will be referred to as (V-T) characteristic. In this case, the minimum time t is fixed to be 50 .mu.sec, while the bias ratio V.sub.2 /(V.sub.2 +V.sub.3) is fixed to 1/3. Waveforms (I.sub.N -S.sub.S) and (I.sub.S -S.sub.S) shown in FIG. 4 are respectively applied to the positive and negative sides of the graph shown in FIG. 8.
Voltages V.sub.1 and V.sub.3 are respectively referred to as "real drive threshold voltage" and "crosstalk voltage", respectively. These voltages meet the condition of V.sub.2 &lt;V.sub.1 &lt;V.sub.3. The difference V=V.sub.3 -V.sub.1 is referred to as a "voltage margin" which represents the voltage width in which the display matrix is drivable. It is considered that the crosstalk voltage V.sub.3 generally exists in driving an FLC display device. Moe practically, the crosstalk voltage V.sub.3 is the voltage at which switching is caused by V.sub.B in the waveform (I.sub.N -I.sub.S) shown in FIG. 4. It is of course possible to increase the value of the voltage V.sub.3 by increasing the bias ratio. Increasing the bias ratio, however, means that the amplitude of the information signal is increased, with the result that the quality of the display image is undesirably degraded due to increase in flicker and reduction in contrast.
The inventors have found that good results are obtained when the bias ratio ranges between 1/3 and 1/4. If the bias ratio is fixed, the voltage margin .DELTA.V strongly depends on the switching characteristic of the liquid crystal material. Obviously, from the view point of ease of drive control of a matrix, it is advantageous that the liquid crystal material used has a large voltage margin .DELTA.V.
As will be understood from the foregoing description, it is possible to write either one of "black` and "white" states in a selected pixel by selective use of the two directions of the information signal, while the non-selected pixel can maintain its "black" or "white" state. The upper and lower limits of the voltage applied as well as the difference therebetween (drive voltage margin .DELTA.V) varies according to the liquid crystal material. In other words, each liquid crystal material has its own upper and lower limit voltage levels and drive voltage margin .DELTA.V. The drive voltage margin also shifts in accordance with a change in environmental temperature, so that display apparatuses must be designed to have an optimum driving voltage for the liquid crystal material used and the expected environmental temperature.
When a display apparatus having a large display area is designed by using this type of matrix display, the variation or difference in the environmental condition, e.g., temperature or cell gap between electrodes, over the liquid crystals of different pixels is increased correspondingly. This means that display image of good and uniform quality cannot be obtained over the large display area when the liquid crystal has only a small drive voltage margin.